This invention relates to the application of protective coatings to the interior seams of cans and, more particularly, to a modular can coater, particularly a relatively small diameter modular can coater, for applying protective coatings to the interior of the welded seams of cans.
Metal cans are generally made by either of one or two processes. One process, the two-piece can process, involves forming a drawn cup from a flat sheet of metal by a blanking process and further forming the cup to a can configuration by an ironing process. The other process, the three-piece process, involves forming a cylindrical can body from a sheet of metal and then attaching two lids to the opposite ends of the body. In the manufacture of three-piece cans, the cylindrical can bodies are formed by wrapping a sheet of metal around a so-called stubhorn. The ends of the sheet are either butted or overlapped and secured together by a welded seam, a soldered seam or a cemented seam. The interior of the seam is then coated with a protective coating which protects the contents of the can against the metal contaminants. The coating is applied to insure that no metal is exposed to the contents of the can. The present invention is directed to apparatus for applying this continuous coating onto can seams.
In a standard production line for the production of cylindrical can bodies by the three-piece process, a stubhorn is provided which acts as a mandrel around which can bodies are formed from a metal blank as they pass downstream over the stubhorn. The can bodies are moved longitudinally over the stubhorn from a magazine by suitable conveyor means such as lugs of a chain conveyor which engage the rear edge of the can bodies and push the can bodies along the stubhorn or a magnetic conveyor wherein moving belts carrying magnets engage the metal cans to move them along the stubhorn. In the final stages of the movement of the can bodies over the stubhorn, the ends of the sheet metal are brought together and joined. The bodies are seamed together by a weld at a welding station. As the bodies pass off the stubhorn and onto rails, they are pushed through an inside striping station. At this station, a stripe of protective material is sprayed over the inside seam of the can. From the striping station, the can body is advanced along a series of rails for further processing such as curing of the coating.
The striping station includes an airless spray apparatus secured to the end of the stubhorn. This apparatus is so positioned that the can bodies pass over it before passing onto to the rails. The spray apparatus is secured to the stubhorn and extends from the downstream end of the stubhorn and includes a nozzle from which the coating material is sprayed along the seam of the can as it passes thereover.
Such can seam coating apparatus exist in commerce today. The flow of coating material through the apparatus is controlled by an air operated valve such that the liquid spray from the coating apparatus is turned on and off in synchronization with movement of the can bodies over the stubhorn. That is, the coating or spray apparatus is activated by the air pressure line extending to the apparatus only when the can seam is passing over the nozzle and is deactivated between cans. For example, a continuously moving line of four-inch long cans may be separated by half-inch gaps. Accordingly, it is necessary to turn the spray apparatus on and off so as not to spray coating material into the gaps. With production lines running at speeds on the order of up to 700 to 750 cans per minute, the cycle rate of the spray apparatus becomes quite high. In known can seam coaters, the air line controlling the coater came in far upstream of the coater on the order of 10 to 12 feet at a minimum. The need to pressurize an air line of this length has resulted in limitations in the cycle rate of the coating apparatus.
There are also can coating systems where the cans are butted end to end during coating to eliminate the gaps between cans so that there is no need to cycle the gun on and off.
Existing can coaters have a diameter on the order of 1 3/4 to 2 inches. With the increasing use of smaller diameter cans, e.g., aerosol cans used in the cosmetics industry, there is a need for a relatively small diameter can coater on the order of 30 mm in diameter. Such a small diameter can coater would be useful both in systems where the gun is rapidly cycled on and off and in systems where it is not.
Likewise, in both types of systems, there is a need for spray apparatus which when secured to the end of the stubhorn can be easily disassembled for maintenance, repair or replacement.